Ion implantation is a physical process that is employed in semiconductor device fabrication to selectively implant dopant into semiconductor and/or wafer material. Thus, the act of implanting does not rely on a chemical interaction between a dopant and semiconductor material. For ion implantation, dopant atoms/molecules are ionized, accelerated, formed into a beam, analyzed, and swept across a wafer, or the wafer is swept through the beam. The dopant ions physically bombard the wafer, enter the surface and come to rest below the surface, at a depth related to their energy.
Ion implanters or ion implantation systems typically include an ion source including an ionization chamber for ionizing a neutral gas from gaseous feed material or from vapor generated from a solid or liquid feed material to produce a collection of charged particles, i.e. ions and electrons, hereinafter referred to as a plasma. Ions are extracted from the ion source plasma by creating an electric field between the source plasma and an electrode, or a series of electrodes, to form an ion beam. The beam travels through specially shaped apertures in each of the electrodes. Typical beam extraction systems are either 3 or 4 electrode systems, although other variations are possible, including very extended systems where final beam deceleration is accomplished in the proximity of the wafer. A standard three electrode system typically comprises a first electrode, the so-called plasma electrode, or arc slit electrode, or ionization chamber electrode, which may be electrically biased at the same large positive voltage as the ion source. This potential determines the beam energy. A second electrode, the so-called suppression electrode, is at a moderate negative voltage to prevent electrons from streaming back to the source chamber. A third and final electrode, the so-called ground electrode, is at ground potential. The extracting electric field is determined by the potential difference between the ionization chamber electrode and the suppression electrode, the details of their shapes, and the distance there between. In many ion implantation processes, it is necessary to generate beams of very different energies and different species for different doping properties, for example 5 kV boron for source-drain extensions and 80 kV arsenic for punchthrough stop. The strength of the electric fields generated by the electrodes must be tuned and adjusted to match the desired extracted beam current and maintain good ion beam optics. This process is called perveance matching. One of the most common methods for achieving good perveance matching is to move the suppression and ground electrodes relative to the ion source, more specifically, the ionization chamber electrode. The distance between the ionization chamber electrode, which defines the ionization chamber aperture, and the suppression electrode, which defines the suppression aperture, is called the extraction gap.
When the ion source is operated with a molecular fill gas or vapor, other species in addition to the desired species for implantation are produced. Some of these species may have very low vapor pressures, and as a result condense on the interior surfaces of the source. These solid deposits may interfere with ion source operation over time, for example by changing the electrical characteristics of the walls or partially blocking the ion source electrode aperture, thereby reducing the available ion current. One method of removing these deposits is the time consuming process of removing the ion source from the system and physically cleaning the source. Another method would be to clean the source in situ by flowing a highly reactive gas through the source, where the gas species is chosen such that the reaction with the deposited material results in a high vapor pressure material which leaves the ion source as a gas and is pumped out of the system. A species with the desired characteristics is atomic fluorine, typically generated by the dissociation of NF3 in a plasma. However, relatively large flow rates of the reactive gas are required to perform proper cleaning of the ion source surfaces. The relatively large flow rates adversely affect both tool cost and cost of operation.